By combining commonly used transfection agents (TA) that have high net positive electrostatic charge with macromolecular high generation (G) dendrimers (G =5, 7, 9, 10) were conjugated to DOTA and Gadolinium (III) ion or Superparamagnetic dextran coated iron oxide nanoparticle (SPIO), effectively alters the NMR relaxation properties of these MRI contrast agents. The physical chemical properties of the TA- contrast agents were modeled and characterized and used to predict the electrostatic interaction between TA and the contrast agents and therefore the combination as a novel method for chaperoning contrast agents into endosomes in cells. Various concentrations of ferumoxides (SPIO) -poly-l-lysine (FE-PLL) complexes were used to magnetically label cells. Rapidly growing cell suspension and adherent cells were effectively labeled via endocytosis into endosomes at low concentrations of ferumoxides (25 ?g/ml media) and PLL (0.75 ?g/ml media). Hematopoietic stem cells and lymphocytes required higher concentrations of PLL (1.5 ?g/ml) in serum free media during initial FE-PLL complex formation prior to label the cells in culture. Total iron concentration in cells depended on the cell type, concentration of FE-PLL complexes in media, cellular density and incubation time. Iron concentrations following overnight incubation with given ferumoxides at 25 ?g/ml media resulted in for example T-cells being labeled with 1-3 pg/cell of intracytoplasmic endosomal iron and 15-20 pg/cell of intracytoplasmic iron in mesenchymal stem cells compared to 0.01 to 0.1 pg/cell for unlabeled cells. No adverse effect on the cell viability and functional capacity or toxicity was observed following magnetic cell labeling with FE-PLL. Biodistribution studies of magnetically labeled human mesenchymal stem cells in rats demonstrated that labeled cells could be detected using a 1.5 Tesla clinical MR unit in the livers for up to 29 days following an intravenous infusion of 900,000 cells. FE-PLL labeled encephalotigenic T-cells were injected into recepient mice to induce experimental allergic encephalomyelitis (EAE) a mouse model of multiple sclerosis. FE-PLL labeled T-cells were detected in the spinal cords of EAE mice at time of initial neurological event using MR microscopy at 7 Tesla. Immmunohistochemical analysis revealed that magnetically labeled T-cells had similar proliferation assays and cytokine profiles as unlabeled encephalotigenic T-cells. In addition, there was excellent correlation between MR microscopy and histology of spinal cords in clinically affected animals. Plans are to translate the magnetic labeling technique using a FDA approved MRI contrast agent Ferumoxides complexed to PLL to label autologous peripherial blood mononuclear cells or stem cells in order to monitor the temporal spatial of these cells for repair, replacement or therapy of central nervous system disease.